This invention relates generally to apparatus for towing underwater instrumentation packages from a surface ship, and, more particularly, to such apparatus used for towing packages of which the elevation has to be accurately controlled or maintained.
In oceanography, it is often important to be able to obtain an accurate profile of the variation of some basic physical parameter, such as temperature, salinity or density of the ocean, in both horizontal and vertical directions. For example, such measurement profiles are very helpful in the study of such oceanic microprocesses as turbulence and turbulent mixing. It has been found that stable stratification in the ocean prevents active turbulance in most of its fluid volume, so that velocity fluctuations, as well as resulting fluctuations of temperature, salinity and density, are often confined to isolated patches. The measurement of these parameters in relation to a fixed spatial frame of reference is, therefore, very important in the study of oceanic turbulance.
The measurement of physical properties of the ocean from a surface vessel has always presented some practical problems. Typically, measurements are made by a sensor package, often referred to as a fish, towed behind the vessel on a line that is weighted, if necessary, to depress the fish to a desired depth below the surface. The most significant practical problem in towing such a package is that the surface motion of the ship is not, of course, purely horizontal, but is affected by wave activity on the ocean surface. Pitching and rolling motions of the ship are transmitted to the fish through the towing line, so that accurate profiling measurements cannot always be obtained.
It will be appreciated from the foregoing that there has been a significant need in the field of oceanography for some means for towing an instrumentation fish in such a manner that the motion of the fish is essentially unaffected by vertical movements of the towing vessel, as may be caused by surface wave motion. The present invention fulfills this need.